Because Christianity equates Satan ("the enemy") with the desires of the body, so satanism accepts that label and chooses to worship everything that Christianity considers a sin as a humanist philosophy. Lust leads to physical and emotional fulfillment, Envy and Greed cause you to take for yourself and improve your condition, Wrath lets you deal with obstacles, Sloth and Gluttony are both fulfilling and are signs that you are accomplished enough to afford them as luxuries. Pride regulates the other sins so you don't commit them to excess.